In the prior art, satellite antennae, terrestrial antennae and integrations of these two have been proposed. Referring initially to the satellite antennae prior art, the quadrifilar helix has been known for several decades. This antenna includes four helical windings fed in phase quadrature. This arrangement provided several characteristics particularly well suited to satellite communications including a hemispherical omnidirectional radiation pattern with excellent circular polarization throughout the radiation pattern as well as compactness and structural simplicity.
For mobile terrestrial communications, the same omnidirectional requirement exists, but the radiation pattern need only to be omnidirectional at the horizon due to the constraints of terrestrial communications on the position of the user relative to base stations. The most common arrangement in the art is the monopole antenna comprising a simple wire above a ground plane.
More contemporary designs of antennae have included dual mode systems. These systems accommodate satellite and terrestrial antennae. These systems present significant design problems particularly with respect to isolation between the two antennae, signal blockage minimization and compactness.
The prior art systems attempted to alleviate the design difficulties by simply placing a satellite antenna and a terrestrial antenna a minimum distance apart such that isolation and blockage requirements were met. Although a generally useful concept, in order to achieve the most desirable performance, a significant separation between the antennae was required. This did not solve the problem of compactness and, in fact, compromised the compactness requirement.
In U.S. Pat. No. 5,600,341, issued Feb. 4, 1997, to Thill et al., there is provided a dual function antenna structure for transceiving in first and second modes.
The apparatus taught in this U.S. patent is a dual frequency single antenna as opposed to a dual mode dual antenna. Accordingly, in the Thill et al. disclosure, there is no teaching with respect to a co-location of two discrete antennae and accordingly, there is no recognition or discussion of the problems encountered when one attempts to co-locate two antennae. The structure provides two feed points for two fields but remains a dual frequency single antenna. This arrangement does not address whatsoever any of the complications inherent in co-location of two antennae such as caging of the signal from antenna to block communication of the co-located antenna.
Further prior art related to the present invention is set forth in U.S. Pat. No. 4,959,657, issued to Mochizuki, issued Sep. 25, 1990. This reference teaches an omnidirectional antenna having a reflector. There is no provision in this reference for the isolation of a monopole antenna with a quadrifilar antenna and accordingly, this reference simply teaches a variation on what is already known in this art.
Moore et al., in U.S. Pat. No. 5,657,792, issued Jul. 22, 1997, discloses a combination GPS and VHF antenna. The combination antenna provides a volute or quadrifilar antenna together with a monopole. Although the elements are provided, there is no co-location between the two antennae which, of course, does not contribute to the compactness of the antenna. By simply providing the combination of the two known antennae in spaced relation, interference problems are not in issue. From a review of the disclosure, it is clear that the Moore et al. reference fails to recognize the value of having a co-located antenna system.
The present invention completely overcomes the limitations in the known art and provides a dual mode antenna system having outstanding performance in a compact system.